The present invention relates to an improvement on a signal interpolation apparatus and a signal interpolation method for eliminating the generation of a false chrominance signal, or more in particular to a signal interpolation apparatus and a signal interpolation method suitable for a single-chip color video camera using a solid-stage image sensor.
A single-chip color video camera for producing a color video signal by use of a solid-state image sensor comprising a combination of mosaic color filters is widely used as a home-use video camera and for other applications. In such a camera, pixels of a solid-state image sensor have corresponding thereto several color filters of different optical pass-bands in periodic fashion. Therefore, the spatial sampling frequency of each chrominance signal is reduced to one half or one third of the sampling frequency of the pixels, with the phases thereof differentiated from each other. As a result, each chrominance signal is liable to be affected by the sideband component generated in the sampling of the pixels, and the problem of a false chrominance signal generated in the edge or the like of an image is posed in a reproduced picture.
A method of reducing the false chrominance signal in a single-chip color video camera is disclosed, for example, in U.S. Pat. No. 4,153,912.
This method will be explained briefly with reference to FIGS. 3 and FIGS. 4A to 4H. FIG. 3 is a diagram showing an example of a conventional color filter for a single-chip color video camera, and FIGS. 4A to 4H a diagram showing output signals produced from a solid-state image sensor comprising a combination of the color filters shown in FIG. 3.
As shown in FIG. 3, the color filter comprises a plurality of filter elements having optical pass-bands of R, G, B arranged repetitively along horizontal direction in stripes, each filter element corresponding to each pixel of the solid-state image sensor. When images with a brightness changing in step along horizontal direction are formed at the same time on this color filter as shown, an output signal shown in FIG. 4B is produced from the solid-state image sensor. The R, G, B signals obtained by separating this output signal for each chrominance signal are shown in FIGS. 4C to 4E. These signals are such that the R signal, for example, is produced but not the G, B signal at time point t1, while the G signal is obtained but not R, B signal at time point t2.
In a conventional method, R1 obtained at time point t1 is compared with R0, R2 produced at time points t-2, t4 before and after a sample to determine which, R0 or R2, is nearer to R1. If R1 is found nearer to R0, the signals G0, B0 obtained before t1 are used, while if R1 is nearer to R2 or near to both R0 and R2 to the same extent, then G1, B1 obtained after t1 are used, so that the G and B signals to be obtained at t1 are interpolated. The R, G, B signals shown in FIGS. 4F to 4H are produced by performing a similar operation at each time point when a given pixel signal is produced. As shown, according to the conventional method described above, the sampling frequency of R, G, B signals is capable of being rendered equal apparently to that of pixels. Also, as obvious from the diagrams, the period when each signal changes with the brightness of the image coincides in the period from t0 to t1, and the phase shift between the signals at the edge of an image is eliminated. As a result, a false chrominance signal which otherwise might be generated at a sharp edge of an image is prevented.
In the case of an image the brightness level of which undergoes a gentle change between t0 and t4 as shown in FIG. 5, for example, an output signal as shown in FIG. 6B is produced from the solid-state image sensor. If this signal is separated into each color signal and processed as described above, the R, G, B signals shown in FIGS. 6F to 6H are obtained. As clear from the diagram, the R signal changes from dark to bright state during the period between t0 and t2, and the G, B signals during the periods between t1 and t3 and between t1 and t4 respectively. The resultant phase shift of the periods of change of each chrominance signal is presented as a false chrominance signal in a reproduced picture.
The problem of the above-mentioned conventional method lies in that although a false chrominance signal may be reduced since the chrominance signals are capable of being changed while kept in phase at an edge of a sharply-changing image, a sufficient improvement is not achieved for an image undergoing a gentle change, thus generating a false chrominance signal.